ObjectId _id <--- index
String UserName
int Points <--- Descending index
Using this document structure as a simple example, we have a collection of users, each with a name and a "points" value. The collection has the usual _id index but also a "descending index" on Points.
The sample use case would be to maintain a ranking scoreboard (something like the League of Legends/DOTA ranking system or chess elo system). Each users' Points field would be constantly changing but the scoreboard is viewed very frequently and thus needs to be accurately maintained.
I'm not sure what "ascending/descending sort order means" in the mongo docs, but apparently it doesn't matter for single-field indices anyways.
So currently I'm just doing a very brute force solution of sorting the collection each time a user's Points field gets updated. At least it's indexed so for a smaller userbase this shouldn't be too bad. However, sorting the entire userbase on each update/insertion just seems wrong in general.
There are data structures traditionally used for maintaining order during insert/update such as search trees but implementing that without putting the entire collection in memory seems like a huge project in itself.
I tried to search for some built-in functionality of Mongo indices that automatically maintains order in the collection for you but I couldn't really find anything like that.
Maybe some logic to only re-sort only some chunk of documents directly above and below the insertion/update? This solution seems pretty dependent on the expected spread of Points across the userbase and the use cases of this system.
Santiago Trujillo
You don't need to sort additionally already created indices , when you create indices in mongoDB you specify in what direction they need to be sorted(ascending(1) or descending(-1)) , so when you search multiple documents based on some field the result will be already sorted based on this field index order. Afcourse you can specify explicitly if you need the result in reverse order or sorted by other field.